Flow rate sensor

ABSTRACT

The invention relates to a flow sensor having a measuring chamber to which a fluid whose volume and/or rate of flow is to be measured is supplied and then drawn off. Inside the measuring chamber elements of a measuring mechanism are mounted so as to freely rotate. The sensor is also provided with a magnet which produces a magnetic field inside the measuring chamber and in close proximity thereto. At least one sensor device measures the magnetic field and/or changes in the magnetic field. The sensor device for measuring the magnetic field is provided with at least one giant magnetoresistance sensor.

TECHNICAL FIELD

The invention relates to a flow rate sensor having a measuring chamber,into which a fluid, the volume and/or flow rate of which is to bemeasured, can be fed and then discharged, having measuring mechanismelements disposed in the measuring chamber and mounted in a freelyrotatable manner, having a magnet for generating a magnetic field in themeasuring chamber and in the immediate vicinity thereof, and having atleast one sensor device for measuring the magnetic field and/orvariations of the magnetic field.

BACKGROUND OF THE INVENTION

Flow rate sensors are also known as volume sensors. Generally, they takethe form of displacement meters. Examples of these are gear sensors,screw spindle meters, oval-wheel meters, cylindrical-piston meters oralternatively measuring turbines or proportioning gear pumps. They areused to measure a volume, a throughflow quantity or the rate at which amedium, here therefore a fluid, passes through the measuring instrument.The fluids may be liquids, pastes or gases.

In practice, flow rate sensors are often not measuring instruments inthe narrower sense because the evaluation electronics are not part ofthe instrument but situated externally. Nevertheless, the term “flowrate measuring instrument” is often used and reference is also made tomeasuring chambers and measuring mechanism elements etc. The flow ratesensors are frequently also described as volume sensors, throughflowsensors, flow rate measuring instruments etc.

The volume sensors or flow rate sensors merely sense the flow or avolume that has flown through and transmit a signal to the evaluationunit or evaluation electronics, which only then produce a measured valuetherefrom. The expression “flow rate sensor” is used below. A confusionwith specific structural elements in the instrument that are detectorsor sensors in the narrower sense is avoided by use of the fulldesignation.

Flow rate sensors or volume sensors in the form of gear sensors areknown for example from EP 0 053 575 B1, EP 0 393 294 A1 or DE 40 42 397C2 as well as EP 0 741 279 B1. They have a housing comprising twohalves. In the one housing half, a pair of circular gear wheels aremounted in a freely rotatable manner in a measuring chamber on fixedaxles by means of ball bearings and without wall contact. The two gearwheels mesh with one another. The medium, the displaced volume or flowrate of which is to be determined, is fed through a first bore to thetwo gear wheels, namely into the region where these gear wheels meshwith one another. The medium therefore passes into the chambers that aremutually formed in the tooth spaces of the two gear wheels. As a resultof the following flow of the medium, the quantities situated in thechambers of the gear wheel are conveyed from the inlet side to theoutlet side and by means of the movement of the teeth then set the gearwheels in rotation. The two gear wheels in said case rotate in oppositedirections. At the other side of the gear wheels in flow directiondownstream of the meshing region, the medium is discharged through asecond bore.

The other housing half serves as a top cover for the region of themeasuring chamber having the two gear wheels plus the medium flowinghere. It therefore tightly closes the measuring chamber and preventsfluid streams from being able to pass through the measuring chamberoutside of the meshing region of the gear wheels. The two housing halveslie one flat on top of the other and between them lies a, so to speak,virtual parting plane.

Permanent magnets such as for example in DE 40 42 397 C2 or carrierfrequency sensors as in EP 0 741 279 B1 are provided in the housingadjacent to the meshing region of the gear wheels and build up anelectromagnetic field. This field is varied by the teeth of the gearwheels and/or by the movement thereof.

The second housing half in the known arrangements mentioned abovemoreover accommodates a magnetoresistive differential sensor. Themagnetoresistive differential sensor senses the variations of the fieldscaused by the movement of the teeth of the rotating gear wheels.

The sensor in these known instruments is separated from the medium orfluid to be measured by a non-magnetic insert, which protects the sensorin particular from the physical and chemical stresses imposed by themedium. The medium may not only have a very different temperature orconsistency but may also be chemically aggressive.

This protection leads disadvantageously to a spacing of the sensor fromthe teeth of the rotating gear wheels that makes measurement moredifficult and limits the accuracy and reliability of measurement. Bymeans of suitable pole pins or other measures, the movement of eachtooth flank during rotation of the associated gear wheel relative to themagnetoresistive differential sensor is then detected and communicatedexternally to suitable evaluation units.

Particularly with larger piece numbers of flow rate sensors, theeconomic aspect gains in importance. Nevertheless, even with largerpiece numbers the accuracy and precision remains important. In manycases, the flow rate of a fluid is also adjusted in closed-loop controlcircuits depending on the measurement.

It would therefore be desirable to carry out as precise as possible ameasurement of the quantity and/or rate of flow of a fluid by means offlow rate sensors that entail the least possible outlay.

The object of the invention is therefore to propose flow rate sensorsthat combine a particularly economical design with nevertheless accuratemeasurement results.

SUMMARY OF THE INVENTION

This object is achieved by flow rate sensors having a measuring chamber,into which a medium (fluid F), the volume and/or flow rate of which isto be measured, can be fed and then discharged, having measuringmechanism elements disposed in the measuring chamber and mounted in afreely rotatable manner, having a magnet for generating a magnetic fieldin the measuring chamber and in the immediate vicinity thereof, andhaving at least one sensor device for measuring the magnetic fieldand/or variations of the magnetic field, wherein the sensor device isdisposed axially offset relative to the freely rotatably mountedmeasuring mechanism elements, and wherein the sensor device formeasuring the magnetic field comprises at least one giantmagnetoresistance sensor.

Giant magnetoresistance, mostly known as GMR effect, is a highlysensitive way of detecting magnetic fields and the changes thereof in amagnetoresistive manner. In connection with flow rate sensors, however,giant magnetoresistance has previously not yet been used.

The use of GMR sensors is known for the scanning of gear wheels in thedifferent connection, say, from DE 296 12 946 U1. The GMR sensor fromthis known arrangement radially scans a tooth of a rotating gear wheel,alternatively the mutually offset teeth of a gear wheel pair disposed onthe same axis. Such arrangements are however unsuitable for volumesensors because in these there always has to be two mutually meshinggear wheels mounted in a narrow housing with close tolerances. Given anarrangement of the GMR sensors as proposed, only the tooth tips arescanned, this leading to an inaccurate and asymmetrical electricalsignal and hence to corresponding and accurate measurements of the flowquantities and flow rates. Furthermore, there is no room laterallyalongside the gear wheels, and/or retro-fitting of the entire system inan installation is considerably impeded.

In an angular resolver according to DE 100 02 331 A1 it is proposed tocarry out as precise as possible a measurement of the angle of arotating part by means of a GMR sensor and a multi-stage gear. The geararrangement alone prevents the use of these ideas in flow rate sensorsand volume sensors.

All of the proposals to fit GMR sensors relate to conventional ambientconditions, i.e. substantially room temperature and normal pressureconditions. Comprehensive application examples in “GMR-Sensors DataBook”, published by the NVE Corporation, Eden Prairie, Minn., USA inApril 2003, likewise relate to normal ambient conditions, possibly withincreased temperatures.

A use of GMR sensors in volume sensors has therefore not yet beenconsidered before because, here, pressures and pressure peaks of 60 Mpato 80 Mpa (600 to 800 bar) may arise and under these extreme loads ahighly precise working method still has to be ensured. What is more, thefluids to be measured may be electrically conductive and aggressiveliquids and this should not impair the serviceability of the entire flowrate sensor.

Surprisingly, by virtue of the concept according to the invention it ispossible to achieve a substantial improvement of conventional flow ratesensors by using GMR sensors in a suitable form. The GMR sensors are nowfitted, not radially outside of the gear wheels as for example in DE 29612 946 U1, but axially directly alongside the gear wheels and/orcomparable elements. In contrast to the background art for flow ratesensors, they may however be disposed there much closer and nearer tothe interior of the measuring chamber adjacent to the teeth.

As the signals of the sensor are markedly stronger and of greatermagnitude than in conventional instruments, they may be processed betterand with less trouble in downstream devices, such as pre-amplifiers orevaluation devices.

It is particularly advantageous that these GMR sensors may beconstructed in the form of integrated circuits (ICs).

In a preferred manner, an integrated circuit of a GMR sensor that issurface-mounted onto an electronic printed circuit board is used. Theprinted circuit board is then on the one hand the electrical connectionof the sensor to the evaluation unit and at the same time seals off theinstrument and/or the measuring chamber mechanically from thepressurized fluids, i.e. liquids or gases, situated in the interior.

As it is possible to dispose the sensor practically in the interior ofthe measuring chamber and/or immediately adjacent thereto, then, unlikein the background art, there is no longer any need for an additionalnon-magnetic shielding, which increases the spatial requirement, and thesensor may therefore be fitted very much closer in towards the region ofthe gear wheels, measuring spindles and the like that are to be scanned.Only the thickness of a casting compound then separates the sensor fromthe measuring mechanism elements that are to be measured. The thicknessof this casting compound is, on the one hand, very low and, on the otherhand, is to be kept very small in dependence upon the external boundaryconditions.

At the same time, the printed circuit board may be used to fix andposition the magnet or magnets. One or more magnets were of course alsorequired already in the background art to build up a magnetic field. Thechanges of the respective magnetic field were, as already mentioned,caused by the movements of the teeth of the gear wheels across themagnetic field. These changes were then conventionally measured by themagnetoresistive differential sensor and evaluated.

A magnetic field is also required according to invention. It isestablished, in the present case too, by means of one or more magnets.However, whereas previously this magnet had to be disposed separatelylikewise in a protected manner simultaneously in the housing, accordingto the invention the printed circuit board, which also carries theintegrated circuits, may likewise receive, fix and/or position themagnet or magnets. The magnetic field built up by the magnet or magnetsis likewise varied by the teeth of the gear wheels and/or by othermeasuring mechanism elements in other forms of flow rate sensors. TheGMR sensor however now detects a different effect, namely an influencingof the giant magnetoresistance by the changing magnetic field.

By virtue of simultaneously using the printed circuit board to fix andposition the magnet or magnets, however, it is possible to dispense withadditional components, thereby allowing an even simpler design of theflow rate sensor.

The printed circuit board may be externally sealed by means of anO-ring.

The printed circuit board itself has plated-through holes, into whichany cables that are required may then be soldered.

This then dispenses with practically all of the structural elements thatare to be additionally installed in the background art for exampleaccording to DE 40 42 397 C2, i.e. the terminal posts or the holdingplate for receiving the posts as well as the no longer requirednon-magnetic insert. The use of the integrated circuit (IC) with theprinted circuit board practically in the pressure chamber and the saidprocedure during installation are extremely economical. Besides theintegrated circuit of the magnet and the printed circuit board,practically no further components are now required.

A further advantage arises in that the Wheatstone bridge that isadvantageously used for evaluation may now be disposed entirely in theinstrument, namely unified with both branches of the Wheatstone bridge.This means that the two branches lie at practically the same temperaturelevel and so it is no longer necessary computationally to effectcompensations at the measured values for differing temperatures in thetwo branches.

For the magnet, in a preferred manner a samarium-cobalt magnet is usedbecause it operates independently of temperature and therefore offersadvantages over magnets constructed on a neodymium base.

The invention may be used not only in gear sensors but also in otherflow rate sensors and/or volume sensors, for example in screw spindlemeters, oval-wheel meters, cylindrical-piston meters or alternatively inmeasuring turbines or in proportioning gear pumps. There, instead of theteeth of gear wheels, measurement is effected by other measuringmechanism elements behaving in an equivalent form, mostly rotating.

Giant magnetoresistance is a specific effect that arises because of achange of the electrical resistance of a multi-layered structure offerromagnetic and non-ferromagnetic layers, which are in each case onlya few nanometres thick. This change of the electrical resistance arisesupon the approach of a magnetic field. Giant magnetoresistance maytherefore actually be utilized in the present case in that, here, thischange is measured in dependence upon the change of the magnetic fieldresulting from the movement of the measuring mechanism elements, i.e.for example from the movement of the teeth of a gear wheel.

The concept according to the invention leads to a high degree ofsensitivity and a large measuring range.

It is i.a. also highly advantageous that by virtue of the invention avery uniform sinusoidal signal may be generated as an output signal.Uniform sinusoidal signals allow particularly reliable evaluation andmoreover a more detailed analysis of the individual sub-regions of thesinusoidal signal, thereby offering further-potential applications.

It is also advantageous that the corresponding sensors for the flow ratesensors according to the invention are very practicable and economicalto manufacture. The printed circuit boards are manufactured in panelsand their components are automatically fitted.

The electrical connection out of the region of the measuring chamber,i.e. out of the pressure chamber, may be effected by means of solderingoperations in the plated-through holes of the printed circuit board.Such a connection is extremely secure and also withstands the very highpressures and thermal loads in the region of the fluids in the measuringchamber.

In said case, it is preferred when the sensor is connected by means of aflat ribbon cable electrically to a circuit device and the flat ribboncable is run from the sensor through a drill hole in the housing of theflow rate sensor to the circuit device.

Said circuit device is in a preferred manner a pre-amplifier. The flatribbon cable then forms the connection from the sensor to thepre-amplifier. The pre-amplifier in turn is a component part of theinstrument according to the invention and transfers the data, which itprocesses, externally to an evaluation unit.

DESCRIPTION OF THE DRAWINGS

An embodiment of the invention is described in detail below withreference to the drawings. The drawings show:

FIG. 1 a vertical section through a flow rate sensor according to theinvention along the line direction A-A of FIG. 2;

FIG. 2 a plan view, partially broken away, of the flow rate sensor ofFIG. 1;

FIG. 3 an enlarged cutout of the detail X of FIG. 1;

FIG. 4 a section through a sub-region of a flow rate sensor according tothe invention; and

FIG. 5 a plan view of the sub-region of FIG. 1;

FIG. 6 a diagrammatic representation of a Wheatstone bridge used inaccordance with the invention.

DETAILED DESCRIPTION

FIG. 1 shows as an overview a section through a flow rate instrumentaccording to the invention. The section does not extend in an exactplane but is offset forward and backward several times in order betterto show the details of the flow rate instrument. The more exact linedirection of the forward and backward offsets may be gathered from FIG.2.

Particularly evident is a housing 10 of the flow rate sensor. Thehousing 10 comprises in particular three plate-like elements laid one ontop of the other, namely a base plate 11, a middle piece 12 and a cover13. In this case, the base plate 11 and the middle piece 12, takentogether, are roughly comparable to the first housing half mentioned inthe background art in DE 40 42 397 C2.

The base plate 11, the middle piece 12 and the cover 13 are connected toone another by fit bolts 16 as well as fastening screws 17. Thisconnection has to be very precise, on the one hand, and very strong, onthe other hand, because a measuring chamber 20 is situated in the innerregion of the middle piece 12.

A short look at FIG. 2 shows the arrangement viewed, in FIG. 1, fromabove. At the right side, the heads of the fit bolts 16 and thefastening screws 17 may be seen. As may be seen, around the top of thecover 13 eight heads of fit bolts and four heads of fastening screws arevisible in order to allow a uniform, strong and sealed tightening ofthese bolts and screws.

Returning to FIG. 1, this reveals that both the fastening screws 17 andthe fit bolts 16 pass right through the housing 10 comprising the baseplate 11, the middle piece 12 and the cover 13.

The measuring chamber 20 is formed only in the middle piece 12 and isaccordingly closed below and above by the base plate 11 and the cover 13respectively, which therefore form the end walls of the measuringchamber 20.

A connection bore 21 in the base plate 11 leads into the measuringchamber 20. Through this connection bore 21 a fluid F, i.e. the mediumin question here, may be fed. A second connection bore in the base plate11 is not visible in FIGS. 1 and 2; through this second connection borethe fluid F is then discharged after running through the measuringchamber 20.

Besides the end walls which, as already mentioned, are formed by thebase plate 11 and the cover 13, the measuring chamber 20 is encircled bythe walls of the middle piece 12, as is also shown in the dashedrepresentation on the left half of FIG. 2. Furthermore, (cf. FIG. 1again) O-rings 22 are provided for sealing the gaps between theunderside and top of the middle piece 12, on the one hand, and theunderside of the cover 13 and the top of the base plate 11.

Two measuring mechanism elements 30 and 40 are situated in the measuringchamber 20. In the illustrated embodiment, these are in each case gearwheels.

In said case, the first gear wheel and/or the first measuring mechanismelement 30 is clearly visible from above in FIG. 2 on the left side.Situated in its centre is the axis 31, about which the gear wheel and/orfirst measuring mechanism element 30 may rotate freely, and a series ofteeth 32 project outwards from the axis 31 of the measuring mechanismelement 30.

In FIG. 1 the first measuring mechanism element 30 may be seen onlydiagrammatically on the left side as the line direction A of the sectionpasses through the measuring chamber 20 only in the edge region of thefirst measuring mechanism element 30.

To make up for this, the second measuring mechanism element 40, heretherefore the second gear wheel, is shown in full section in FIG. 1. Inthe section, the axis 41 and in addition two flanks of teeth 42 may beseen. Also evident are various elements of a bearing 43, which ensuresthe freedom to rotate of the second measuring mechanism element 40 too.

Both measuring mechanism elements 30, 40 are made of a ferromagneticmaterial, by means of which magnetic fields may be markedly influencedwhen the measuring mechanism elements 30, 40 rotate about the axes 31,41.

As may be seen, the two measuring mechanism elements 30, 40 mesh withone another and the fluid F fed through the connection bore 21 givesrise to a rotation of the two measuring mechanism elements 30, 40 inopposite directions.

A sensor device 50 is represented relatively small in FIG. 1. From FIG.2 it is evident that in the concrete embodiment two sensor devices 50 ofsimilar design are provided. Both are situated above the measuringchamber 20 in a region, below and across which the teeth 32 of the firstmeasuring mechanism element 30 rotate. Because of the ferromagneticproperties of the first measuring mechanism element 30, a magnetic field56 situated below the sensor device 50 is influenced and changes.Details of this are additionally indicated below.

One of the two sensor devices 50 is shown to a slightly enlarged scalein FIG. 3. FIG. 3 therefore shows a sub-region of the cover 13 above themeasuring chamber 20. Cut out in the cover 13 is a channel, in which thesensor device 50 is fitted.

Central element of the sensor device 50 is a magnet 55, here a roundmagnet. It builds up the magnetic field 56 that is varied by theferromagnetic properties of the measuring mechanism element 30.

The changes of the magnetic field are picked up and acquired by a sensor52 that operates on the basis of the physical effect of giantmagnetoresistance. This sensor 52 is disposed almost directly above thebottom edge of the cover 13 and therefore lies almost without clearanceabove the measuring chamber 20, in which the first measuring mechanismelement 30 rotates. The changes of the magnetic field 56 therefore occurpractically immediately next to the sensor 52 and may be picked up in ahighly precise and exact manner.

The sensor 52 and the magnet 55 are both disposed on a printed circuitboard 60 and connected thereto. Also situated on this printed circuitboard 60 is an integrated circuit (not shown). The printed circuit boardis simultaneously a pressure plate. It is sealed on all sides inside thecover 13 by an O-ring 61 because in the measuring chamber 20 situatedimmediately below the sensor 52, as already mentioned, there are fluidsthat may have very high temperatures. There, moreover, a high pressuremay prevail and the fluids F may be chemically or physically aggressive.The pressure plate property of the printed circuit board 60 togetherwith the sealing by the O-ring 61 prevents the fluid F from penetratinginto the cover 13 behind the printed circuit board 60, viewed in FIG. 3or in an upward direction in FIG. 1.

An electrical connection of the printed circuit board 60, the magnet 55and the sensor 52 having the GMR-measuring properties is effected bymeans of a flat ribbon cable 62, which is not diagrammaticallyrepresented here.

The region around the flat ribbon cable 62 is filled by a castingcompound 65 in order to keep the cable completely stable and prevent thepenetration of foreign bodies from outside of the housing into thisregion.

A second casting compound 66 entirely fills the region between theprinted circuit board 60 having the pressure plate properties and themeasuring chamber 20 and therefore completely embeds the sensor 52.Here, a smooth surface is desired in order to rule out any flowbehaviour of the fluid F that might interfere with the measurement.

From FIG. 1 it is evident that the connection by means of the flatribbon cable 62 leads into an intermediate plate 71, through which thereis a connection to a pre-amplifier 72 and, from there, out of the flowrate sensor to an evaluation unit 73. These are represented here purelydiagrammatically. They may optionally be exchanged and adapted to theconcrete external conditions of the flow rate sensor.

The magnet 55, in a preferred form of construction a samarium-cobaltmagnet, generates the magnetic field 56. This magnetic field 56 extendsinto the measuring chamber 20 and the surrounding regions adjacentthereto. The magnetic field 56 penetrates in particular the sensor 52,which here according to the invention is a GMR sensor. As a result ofthe rotation, the magnetic field 56 is perturbed by the adjacent tooth32 running past just below the magnet 55 and the sensor 52 and by theassociated tooth space of the gear wheel 30. This varying magnetic field56 generates in the GMR sensor 52 an electrical signal, which in theswitching device, thus here the pre-amplifier 72, is amplified anddigitized. The digital signal is then transmitted via a further cable(not shown) to the evaluation electronics outside of the flow ratesensor and is evaluated there.

In FIG. 4 the illustration of FIG. 3 is repeated once more in a similarform. Here, for illustrative reasons, the view has practically beenturned upside down so that the measuring chamber 20 of the flow ratesensor in the housing 10 is situated at the top. Also indicated there isthat in this region the magnetic field 56 that is regularly changed bythe movement of the teeth 32 (not shown) of the first measuringmechanism element 30 is situated. Here, it should moreover be taken intoconsideration that, should the measuring mechanism element not be a gearwheel, other elements instead of teeth are conceivable.

The magnet 55 and the sensor 52, which utilizes the giantmagneto-resistance effect, are illustrated once more to an enlargedscale.

The magnet 55 and the further elements are electrically connected by aflat ribbon cable 62, running in a downward direction in theillustration in FIG. 4, to the intermediate plate 71 and the furtherelements described in connection with FIG. 3.

FIG. 5 once more shows the view of FIG. 4, namely in this case viewedfrom above. The view is therefore onto the casting compound 66.Additionally indicated is a tooth 32 of the measuring mechanism element30 that is situated precisely below the sensor device 50, i.e. issituated in a movement, in which it sweeps past this region.

Notionally in the illustration the casting compound 66 is transparent,which in practice naturally need not be the case. It is thereforepossible in the present case to see the sensor 52 through the castingcompound 66, and moreover run connections so that the printed circuitboard 60 is partially visible.

FIG. 6 is a diagrammatic representation of the structure of a Wheatstonebridge, which as part of the integrated circuit on the printed circuitboard 60 includes the GMR sensor 52.

What may be seen is the conventional circuit of a Wheatstone bridehaving four resistors, of which three are known and the fourth iscorrespondingly influenced by the magnetic field 56.

REFERENCE CHARACTERS

-   10 housing-   11 base plate-   12 middle piece-   13 cover-   16 fit bolts-   17 fastening screws-   20 measuring chamber-   21 connection bore-   22 O-rings for the measuring chamber-   30 first measuring mechanism element, in particular first gear wheel-   31 axis of first measuring mechanism element 30-   32 tooth of first measuring mechanism element 30-   40 second measuring mechanism element, in particular second gear    wheel-   41 axis of second measuring mechanism element 40-   42 tooth of second measuring mechanism element 40-   43 bearing arrangement of second measuring mechanism element 40-   50 sensor device-   52 sensor-   55 magnet-   56 magnetic field-   60 printed circuit board-   61 O-ring-   62 flat ribbon cable-   65 first casting compound-   66 second casting compound-   71 intermediate plate-   72 pre-amplifier-   73 evaluation unit-   F fluid

1. Flow rate sensor including a measuring chamber, into which a medium,the volume and/or flow rate of which is to be measured, can be fed andthen discharged, said flow rate sensor comprising: including measuringmechanism elements disposed in the measuring chamber and mounted in afreely rotatable manner, a magnet for generating a magnetic field in themeasuring chamber and in the immediate vicinity thereof, at least onesensor device for measuring the magnetic field and/or variations of themagnetic field, wherein the sensor device is disposed axially offsetrelative to the freely rotatably mounted measuring mechanism elements,wherein the sensor device for measuring the magnetic field comprises atleast one magnetoresistance sensor, wherein the sensor device includes acircuit board which carries the at least one sensor, and wherein circuitboard carries the magnet.
 2. Flow rate sensor according to claim 1,wherein the circuit board comprises a printed circuit board, which isconfigured as a pressure plate, and the sensor device is mounted at oneside of the pressure plate and the magnet is mounted at an opposite sideof pressure plate.
 3. Flow rate sensor according to claim 2, wherein themeasuring mechanism elements comprise a pair of intermeshing gears thesensor device comprises a pair of spacedly disposed sensors that areconstructed and arranged relative to only one of the gears.
 4. Flow ratesensor including a measuring chamber, into which a medium, the volumeand/or flow rate of which is to be measured, can be fed and thendischarged, said flow rate sensor comprising: measuring mechanismelements disposed in the measuring chamber and mounted in a freelyrotatable manner, a magnet for generating a magnetic field in themeasuring chamber and in the immediate vicinity thereof, at least onesensor device for measuring the magnetic field and/or variations of themagnetic field, wherein the sensor device is disposed axially offsetrelative to the freely rotatably mounted measuring mechanism elements,wherein the sensor device for measuring the magnetic field comprises atleast one magnetoresistance sensor, wherein the sensor is connected bymeans of a flat ribbon cable electrically to a circuit device, and thatthe flat ribbon cable is run from the sensor through a housing of theflow rate sensor to the circuit device.
 5. Flow rate sensor according toclaim 2, wherein the printed circuit board is disposed immediatelyadjacent to the measuring chamber and is separated from the measuringchamber only by a casting compound.
 6. Flow rate sensor according toclaim 5, wherein the magnet is a samarium-cobalt magnet.
 7. Flow ratesensor according to claim 2, wherein the sensor is connected by means ofa flat ribbon cable electrically to a circuit device, and that the flatribbon cable is run from the sensor through a drill hole in the housingof the flow rate sensor to the circuit device.
 8. Flow rate sensoraccording to claim 3, wherein the sensor is connected by means of a flatribbon cable electrically to a circuit device, and that the flat ribboncable is run from the sensor through a drill hole in the housing of theflow rate sensor to the circuit device.
 9. Flow rate sensor according toclaim 3, wherein the printed circuit board is disposed immediatelyadjacent to the measuring chamber and is separated from the measuringchamber only by a casting compound.
 10. Flow rate sensor according toclaim 4, wherein the printed circuit board is disposed immediatelyadjacent to the measuring chamber and is separated from the measuringchamber only by a casting compound, that the flat ribbon cable is runfrom the sensor through a drill hole in the housing of the flow ratesensor to the circuit device.
 11. Flow rate sensor according to claim 1,wherein the magnet is a samarium-cobalt magnet.
 12. Flow rate sensoraccording to claim 2, wherein the magnet is a samarium-cobalt magnet 13.Flow rate sensor according to claim 3, wherein the magnet is asamarium-cobalt magnet
 14. Flow rate sensor according to claim 4,wherein the magnet is a samarium-cohalt magnet.
 15. Flow rate sensoraccording to claim 1, wherein the sensor is connected by means of a flatribbon cable electrically to a circuit device, and that the flat ribboncable is run from the sensor through a drill hole in the housing of theflow rate sensor to the circuit device; wherein the printed circuitboard is disposed immediately adjacent to the measuring chamber and isseparated from the measuring chamber only by a casting compound; andwherein the magnet is a samarium-cobalt magnet.
 16. Flow rate sensoraccording to claim 1, wherein the sensor device comprises a pair ofspacedly disposed sensors.
 17. Flow rate sensor according to claim 16,wherein the measuring elements comprise a pair of engaged measurementelements each having an axis of rotation.
 18. Flow rate sensor accordingto claim 17, wherein each sensor device is disposed axially offsetrelative to the freely rotatably mounted measuring mechanism elements.19. Flow rate sensor according to claim 18, wherein the measuringmechanism elements comprise a pair of intermeshing gears and the pair ofspacedly disposed sensors that are constructed and arranged relative toonly one of the gears.
 20. Flow rate sensor including a measuringchamber, into which a medium the volume and/or flow rate of which is tobe measured, can be fed and then discharged, said flow rate sensorcomprising: a pair of measuring mechanism elements disposed in themeasuring chamber and each mounted in a freely rotatable manner aboutrespective rotation axes; a magnet for generating a magnetic field inthe measuring chamber and in the immediate vicinity thereof; a pair ofspacedly disposed sensor devices for measuring the magnetic field and/orvariations of the magnetic field; wherein both of the sensor devices aredisposed axially offset relative to the rotation axes of only one of thefreely rotatably mounted measuring mechanism elements; and wherein eachsensor device for measuring the magnetic field comprises at least onemagnetoresistance sensor.
 21. Flow rate sensor according to claim 20,wherein the measuring mechanism elements comprise a pair of intermeshinggears, and the spacedly disposed sensor devices are arrangedcircumferentially about one of the intermeshing gears.
 22. Flow ratesensor according to claim 20, including a circuit board which carriesthe pair of sensor devices, and wherein the circuit board also carriesthe magnet.
 23. Flow rate sensor according to claim 22, wherein thecircuit board is configured as a pressure plate and the sensor device ismounted at one side of the pressure plate and the magnet is mounted atan opposite side of the pressure plate.